Simplify the following expression: $4\sqrt{20}$.
Answer: First, try to factor a perfect square out of the radical. $= 4\sqrt{20}$ $= 4\sqrt{4 \cdot 5}$ Separate the radical and simplify. $= 4\sqrt{4} \cdot \sqrt{5}$ $= 4 \cdot 2 \cdot \sqrt{5}$ $= 8\sqrt{5}$